Tertiary air conditioning sizing: power calculation per m2
Applicable regulatory framework and the real 80 to 130 W/m2 range
80 to 130 W/m2: this is the real cooling power range for tertiary offices, far from the 50 W/m2 flat rate still frequently used at the preliminary design stage. This flat rate, inherited from 1990s offices, underestimates current IT density (10 m2/workstation) and south-west solar gains. The AICVF hourly calculation method significantly reduces the gap between detailed design and delivery, while optimising electricity consumption in operation. The AICVF method and the tertiary decree frame the approach.
Calculating air conditioning power in offices relies on an analysis of the building and its occupants. The standard applicable to the tertiary sector sets the criteria for ambient conditions and air quality, with a target operative temperature of 24 to 26 °C in cooling mode. At the same time, employment regulations require 25 m³/h/person of fresh air in standard offices and 30 m³/h/person in meeting rooms, a constraint that directly affects the sizing of the air handling unit.
Two families of thermal gains structure the assessment:
- Internal gains: occupants, lighting in W, computer workstations, servers, printers.
- External gains: south-west and north-east orientations, ceiling and floor insulation, glazed surfaces with or without external blinds.
Our reading differs from common practice on one specific point: the professional consensus still treats IT density as a secondary variable, whereas it has become dominant in flex offices. On the offices we deliver, occupancy density tends to settle around 10 m2 per workstation in flex offices, compared with 12 to 15 m2/workstation in older sector standards, a gap that directly affects the thermal assessment. The average power of 50 W/m2 in flat-rate sketch-stage estimates remains an indicative value to be checked against a detailed hourly assessment. Oversizing by 20 to 30 % degrades the seasonal COP of reversible direct-expansion heat pumps. On the HVAC projects we have supported since 2006, a significant proportion of the detailed designs received show overestimated cooling power, due to the lack of an hourly assessment.
AICVF method in 4 verifiable steps over 12 weeks
The Kytom sizing method follows 4 verifiable steps, deployed over the average 12-week timeframe of fit-out projects, with an hourly thermal assessment structured by zone.
- Geometric survey and envelope qualification: measurement of glazed surfaces, identification of south-west and north-east orientations, checking of ceiling and floor insulation.
- Calculation of internal gains: actual number of occupants, installed lighting power in W, IT load (workstations, screens, servers).
- Simplified or detailed thermal assessment depending on complexity, producing a W/m2 ratio by zone (open space, meeting room, server room).
- Choice of reversible system and zoning: target of 24 to 26 °C at peak, in order to preserve the seasonal COP of tertiary heat pumps.
| Zone type | Target ratio (W/m2) | Reference |
|---|---|---|
| North-facing open space, blinds | 85-95 | AICVF thermal assessment |
| South-west-facing open space | 110-125 | AICVF thermal assessment |
| High-density meeting room | 130-160 | Detailed thermal assessment |
| Server room | 300-500 | Measured project IT load |
Triangulating the surface ratio, the assessment by zone and feedback from comparable buildings significantly reduces the sizing gap between detailed design and delivery.
When this approach is not the right one: a full hourly assessment is not justified below 200 m2 or for a single-zone space with homogeneous gains (non-glazed room, stable occupancy). The cost of studies then exceeds the gain in avoided power, and a simplified assessment or an orientation-and-density ratio drawn from a documented benchmark is sufficient. For a building destined for major reconfiguration within 3 years, intentionally oversizing by 10 to 15 % becomes more cost-effective than a fine calculation that new uses will render obsolete.
For the thermal engineer: reduced electricity consumption and a trajectory aligned with tertiary stock obligations
Reframed from the professional perspective of the thermal engineer and the energy manager, sizing is not an isolated detailed design deliverable: it is the entry point to the energy efficiency trajectory imposed on the tertiary building stock. Precise sizing produces three concrete effects on our HVAC projects.
In CAPEX, the installed cooling power is significantly lowered compared with initial flat-rate estimates, reducing equipment costs on standard office areas. In OPEX, the seasonal COP of reversible heat pumps improves, resulting in a noticeable drop in electricity consumption from the first year of operation. In comfort, post-delivery occupancy feedback reports far fewer thermal complaints on sites sized with a detailed thermal assessment.
An adjusted calculation improves the seasonal performance of tertiary heat pumps and significantly reduces occupants’ thermal complaints, particularly when sizing is based on a detailed thermal assessment rather than a flat-rate ratio.
Acoustic comfort also enters the equation: environmental standards for tertiary buildings cap technical equipment (air handling units, fan coil units) at 38 dB(A) in enclosed offices, a constraint that guides the choice of terminals and the supply air speed.
4 points to watch: flex office, south-west glazing, server rooms
Air conditioning power calculation involves 4 recurring points to watch.
Technical rooms (server rooms) require specific sizing of 300 to 500 W/m2 depending on the installed IT load, with N+1 redundancy and a thermostat separate from the office zone. Combining the IT zone and the occupant zone within the same chiller frequently leads to premature replacement of the system.
South-west glazing without external solar protection generates peak summer solar gains that can reach 350 to 500 W/m2 of glazing (AICVF standard), justifying external blinds or selective films to avoid oversizing the heat pump.
In flex offices, the densification of workstations significantly increases the internal gains to be anticipated from the detailed design stage, failing which the installation will be undersized.
Meeting rooms, lightly occupied on average but saturated at peak, require independent zoning with presence detection to avoid unnecessary continuous air conditioning.
